In the past, soldering has generally required flux to promote wetting and good solder joints. Flux removes surface oxides on the metallic surfaces to be soldered. When elements, such as printed wiring boards and the like are soldered, they are generally fluxed prior to soldering. More recently, inert gas soldering, generally nitrogen gas, in some cases may also be mixed with other types of gas, have been used to replace fluxes. By eliminating flux and still preventing oxidation of the solder, one avoids the necessity of cleaning the solder joints after soldering. Reducing agents and other soldering additions, in gaseous form, liquid droplets or mist, or in some cases in powder form, are sometimes applied even in fluxless soldering.
In one embodiment, soldering is achieved by passing an element through a solder wave. In another embodiment reflow soldering is used wherein solder paste is first applied to the metallic components to be soldered, and the element is then heated so that the solder melts to liquid solder which wets and forms the solder joint. Most solder paste used for reflow soldering includes flux. However if reflow soldering occurs in an inert atmosphere, then a reduction of the quantity of flux in the solder paste can be achieved.
In our co-pending application filed Dec. 6, 1989 entitled Tunnel for Fluxless Soldering, Ser. No. 448,008 is disclosed soldering of elements, such as printed wiring boards in a non-explosive gas atmosphere which substantially excludes oxygen. Disclosure of this application is incorporated herein by reference.
The tunnel enclosure disclosed in our co-pending application is provided with entry gas curtains and exit gas curtains at the entry and exit to the tunnel. These gas curtains may be of the type shown in U.S. Pat. No. 4,696,226 to Witmer and provide two sets of curtains at both the entry and exit with a space between, which is sufficient to permit the element to be soldered to pass completely through the first curtain without interrupting the flow of fluid at the second curtain. When the element passes through the second curtain, then the flow of fluid to the first curtain is not interrupted. In this way, one of the gas curtains is always closed ensuring that no gas within the enclosure escapes out of the tunnel enclosure and no air from outside enters the enclosure.
It is an aim of the present invention to provide at least one enclosed zone for soldering which has at least one fluid barrier curtain at both the entry and the exit and permits the addition of air additive such as a soldering aid, to be introduced into the enclosed zone by means of the fluid barrier curtain. It is a further aim to provide multiple enclosed zones with fluid barrier curtains between zones wherein the zones can be at different temperatures, contain different gases or percentages of gas combinations, or have different ambient conditions.